A reduction projection exposure apparatus using ultraviolet (UV) light is currently used in leading-edge lithography for manufacturing a fine semiconductor element of a memory or logic circuit. Along with an increase in the degree of integration of a semiconductor element, the element often has a line width of 50 nm or less, but lithography using UV light may not be able to provide a sufficient resolving power. To combat this situation, lithography using EUV (Extreme Ultraviolet) light, with a shorter wavelength of 11 to 15 nm, is under development.
Unlike UV lithography, EUV lithography cannot use a transmissive optical element using refraction, because EUV light exhibits high absorbance. EUV lithography thus uses a reflective optical element, the surface of which has a multilayer film in which two types of materials having different optical constants are alternately stacked. EUV light having a wavelength of 13.5 nm or in its vicinity is reflected by a reflective optical element having a glass-based substrate surface, which is polished into a precise shape, and in which molybdenum (Mo) and silicon (Si) are alternately stacked efficiently. An EUV exposure apparatus causes a reduction projection optical system including such a reflective optical element to form a mask pattern image on the wafer.
Since the ambient gas component also absorbs EUV light, the exposure apparatus maintains its interior in an appropriate vacuum state. Gas containing moisture and a carbonaceous substance remains in that vacuum atmosphere. The gas containing these substances mainly results from the parts or cables used in the exposure apparatus. The gas also contains a component which vaporizes from the resist (photosensitive material) applied on the wafer. The surface of the optical element adsorbs these residual gas components. Throughout the duration of stay on that surface, EUV light strikes the residual gas components. Since EUV light also has a high substance decomposition ability, it decomposes the carbonaceous residual gas component on the surface into carbon, which deposits on that surface. Similarly, the EUV light decomposes the moisture on the surface into active oxygen, which oxidizes the surface material. The presence of carbon and oxygen on the surface allows it to absorb the EUV light in amounts which depend on their thicknesses. The absorption amount is non-negligible, and the reflectance of the optical element to EUV light deteriorates. A reduction projection optical system having an excellent resolving power requires a large number of optical elements. Therefore, deterioration in reflectance per element synergistically decreases the amount of light which passes through the optical system. This prolongs the exposure time, resulting in degradation in performance of the exposure apparatus.
There have been proposals for preventing carbon deposition and removing the carbon deposits up to now. To prevent carbon deposition, it is a common practice to decrease the pressure of the target gas in the atmosphere in which the optical element is placed. The target gas is, e.g., a hydrocarbon, such as methane, ethane, or propane, a straight-chain organic substance, such as isopropyl alcohol or polymethyl methacrylate, or a cyclic organic substance such as phthalate ester (Japanese Patent No. 3467485).
There is a proposal for removing deposited carbon in the following method (Japanese Patent Laid-Open No. 2003-188096). This method irradiates the deposited carbon with UV light or EUV light having a wavelength shorter than 250 nm in an environment in which an oxygen-containing species selected from water, nitrogen oxides, and oxygen-containing hydrocarbons exists in the atmosphere in which the optical element is placed. There is also a proposal for removing the carbon in the following method (Japanese Patent Laid-Open No. 2002-237443). This method introduces a processing gas containing at least one of the substances selected from the group consisting of a gas of water, oxygen, ozone, and a radical element, and irradiates the carbon with an electron beam.
A popular approach effective in preventing oxidation to some extent is to perform exposure after introducing ethanol in the atmosphere in which an optical element having a multilayer film, the uppermost layer of which is Si, is placed (L. Klebanoff, 2nd International EUVL Workshop, [online], Oct. 19-20, 2000, International SEMATECH Manufacturing Initiative, [searched Jun. 10, 2005, Internet http://www.sematech.org/resources/litho/meetings/euvl/20001019/index/htm>). Also, a popular approach, which can suppress EUV light absorption to some extent, is to form a ruthenium (Ru) protective film on the uppermost layer of the multilayer film, because Ru has an anti-oxidative property (S. Bajt, H. Chapman, N. Nguyen, J. Alameda, J. Robinson, M. Malinowski, E. Gullikson, A. Aquila, C. Tarrio, S. Granthan, SPIE, 2003, Vol. 5037, p. 236).